This invention relates to a frequency modulation (FM) signal demodulating circuit and more particularly, to a demodulating circuit employing pulse counting.
Television broadcasting stations in Japan broadcast monophonic programs, stereophonic programs and bilingual programs. In the case of the monophonic program, only a primary carrier (4.5 MHz) is modulated by the frequency of a monophonic signal; its FM monophonic signal is superimposed upon a composite video signal. In the stereophonic program, a subaudio signal and a pilot signal (983.5 Hz), in addition to a main audio signal, are employed. A subcarrier (approximately 31.5 KHz) is modulated by the frequency of the subaudio signal to the extent of .+-.10 KHz. A control signal (55.125 KHz) is modulated by the amplitude of the pilot signal. The FM audio subcarrier signal and the AM control signal are superimposed upon the main audio signal to produce a composite signal. The main carrier is modulated by the frequency of this composite signal, and the resulting FM composite audio signal is superimposed upon a composite video signal.
In the case of the bilingual program, a pilot signal (922.5 Hz) is employed. This pilot signal is used, as in the case of the stereophonic program, to amplitude modulate the control signal. The main audio signal and the subaudio signal are utilized as in the case of the stereophonic program to produce the FM composite audio signal.
Various methods are known in the prior art to demodulate the FM audio subcarrier signal included in the composite audio signal. For example, either the pulse counting method or the phase-locked loop (PLL) method can be employed. A conventional FM signal demodulating circuit employing pulse counting is shown in FIG. 1. FIG. 2 shows waveforms of the signals at each point of the circuit of FIG. 1. As shown, an FM subcarrier signal (FIG. 2a) is supplied to a limiter 11 through an input terminal 10. The FM signal is limited by limiter 11 in order to remove any noise included in the FM signal. The limited FM signal (FIG. 2b) is supplied from a pair of output terminals to a differential circuit 12. Circuit 12 comprises a serially connected resistor R.sub.11 and the primary winding of transformer T.sub.11 which are connected between output terminals of circuit 11. The limited FM signal is differentiated by circuit 12 which produces the differentiated FM signal shown in FIG. 2c. The differentiated signal is supplied from the secondary end taps of transformer T.sub.11 to the respective cathode electrode of diodes D.sub.11 and D.sub.12. The secondary's center tap is grounded. Diodes D.sub.11, D.sub.12 function as a full-wave detector and produce the detected signal as shown in FIG. 2d. That is, a demodulated subaudio signal is produced at the connection point of diodes D.sub.11, D.sub.12. This demodulated signal is supplied to an integrating circuit 13 and to a resistor R.sub.12 which is connected between the anode of diode D.sub.11 and ground. Integrating circuit 13 is a series circuit, connected across resistor R.sub.12, comprising a resistor R.sub.13 and a condenser C.sub.11. Circuit 13 transforms the demodulated signal into the analog subaudio signal (FIG. 2e) necessary for driving a speaker. This subaudio signal is supplied from a connection point of resistor R.sub.13 and condenser C.sub.11 to an output terminal 14. Prior art circuits which include a transformer, such as T.sub.11, have the disadvantage that they can not be fabricated into a semiconductor integrated circuit.
In addition, a conventional FM signal demodulating circuit employing the PLL method is shown in FIG. 3. As shown, an FM subcarrier signal is supplied to a limiter 21 through an input terminal 20. THe limited FM signal is supplied to a PLL 22. PLL 22 comprises a phase comparator 23, a low-pass filter (LPF) 24, a direct current (DC) amplifier 25 and a voltage-controlled oscillator (VCO) 26. The limited FM signal and an oscillation signal from the VCO 26 are supplied to input terminals of comparator 23. An error signal between the limited FM signal and the oscillation signal (i.e., a demodulated, subaudio signal) is produced by comparator 23. This demodulated signal is transformed into an analog subaudio signal by LPF 24. The analog subaudio signal, after amplification by DC amplifier 25, is supplied to an output terminal 30. The output of DC amplifier 25 is also fedback to VCO 26 in order to control its oscillation frequency. In operation, PLL 22 is synchronized with the frequency of the limited FM signal; consequently, the oscillation frequency of VCO 26 varies in accordance with the frequency of this limited signal. This is shown by the characteristic graph of the demodulating circuit in FIG. 4. The abscissa indicates the frequency of the FM audio subcarrier signal, and the ordinate indicates the demodulated output voltage. The capture range for this circuit is shown to be from 21.5 KHz to 41.5 KHz.
This prior art circuit, however, has the following disadvantages: (1) When the FM subaudio signal is overmodulated over the capture range, distortion of the demodulated output signal is produced; (2) Since PLL 22 includes a low pass filter (LPF 24), a delay of the demodulated output signal occurs, accordingly, a time correction means for the delayed output signal is necessary; (3) Such demodulating circuits are complicated and costly to manufacture; accordingly, integrating it into a semicocductor chip requires many outside connection pins and peripheral devices (e.g., capacitors).